The Illinois senator used his first public appearance of the week to knock down the notion that he might accept the party's vice presidential spot on the fall ticket. He noted that he has won more states, votes and delegates than Clinton so far.
"I don't know how somebody who is in second place is offering the vice presidency to someone who is first place," Obama said, drawing cheers and a standing ovation from about 1,700 people in Columbus, Miss.
Clinton aides say she's just reflecting talk that's already out there, reports CBS News correspondent Jim Axelrod. The Obama camp says that Clinton would make a terrific VP as well but that they just don't want to get ahead of themselves.
Saying he wanted to be "absolutely clear," Obama added: "I don't want anybody here thinking that 'Somehow maybe I can get both"' by nominating Clinton as president and assuming he would be her running mate. "You have to make a choice in this election," he said.
After suffering losses in Texas, Ohio and Rhode Island, the Obama campaign promised to aggressively respond to criticisms from Clinton, reports CBS News' Maria Gavrilovic. Monday was the first time that Obama responded to suggestions by Clinton and former President Bill Clinton of a Clinton-Obama ticket, which he took seven minutes to do. (read more from Gavrilovic in the CBS News "From The Road" blog).
Obama aides said Clinton's recent hints that she might welcome him as her vice presidential candidate appeared meant to diminish him and to attract undecided voters in the remaining primary states by suggesting they can have a "dream ticket."
"Neither candidate wants to be seen as second fiddle but it's even more perplexing when the leader in the nomination battle is put into that role by the candidate who is trailing in the delegate count," said CBSNews.com senior political editor Vaughn Ververs. "Obama refuted the suggestion effectively but Clinton may have accomplished what she set out to do, and that's plant the idea into the minds of whose uncommitted super delegates who may well decide this nomination."
Obama had never suggested he might accept a second spot on the ticket. But until Monday he had not ridiculed the notion so directly, even if he did completely rule it out in Shermanesque terms.
He told the audience that it made no sense for Clinton to suggest he is not ready to be president and then hint that she might hand him the job that could make him president at a moment's notice.
"If I'm not ready, how is it you think I would be such a great vice president?" he said, as the crowd laughed and cheered loudly.
Mississippi holds it primary Tuesday, the last contest before the Pennsylvania primary six weeks from now.
Clinton and her husband, the former president, had suggested recently that a Clinton-Obama ticket would be popular and formidable against Republican Sen. John McCain in November.
Many political activists discounted the notion all along. They noted that the two senators lack a warm relationship and, more important, that Obama would be ill-served by hinting he might accept the vice presidential slot when he holds the lead in delegates and hopes to win the presidential nomination.
In the latest CBS News count, Obama leads Clinton, 1,570-1,461. He has won 28 contests to her 17.
Moreover, many insiders feel the ambitious and fast-rising Obama would chafe in the vice president's job, especially in a White House where Bill Clinton would almost surely play a huge advisory role.
Still, the notion of a Clinton-Obama ticket has received ample discussion in recent days on cable TV news shows and newspapers such as New York City's tabloids.
In an interview Friday in Wyoming with KTVQ-TV, a CBS affiliate based in Billings, Mont., Obama's comments were somewhat mixed.
"Well, you know, I think it's premature," he said of accepting the second spot on the ticket. "You won't see me as a vice presidential candidate."
His Monday remarks were more detailed, pointed and humorous.
Of course, they will not completely end the speculation. Presidential candidates routinely disavow any interest in the vice presidential spot. But some, including John Edwards and Al Gore, change their minds when they fall short of their top goal.